$200–$2,400
DIY from $35
Moderate
If you do it yourself
Within a few weeks
Before bigger damage
Common symptoms
- Check engine light is on, usually steady rather than flashing
- Often no change in how the car drives
- Will likely fail an emissions or smog test
- Slightly worse fuel economy in some cases
- Rotten-egg sulfur smell from the exhaust when severe
Common causes, ranked by likelihood
Based on typical diagnoses for P0420. Work from the most likely cause down before replacing expensive parts.
The most common cause on higher-mileage vehicles.
Can damage the converter over time.
How to diagnose it
- Scan for other codes first and fix any misfire (P0300-series) or fuel-trim codes (P0171/P0174) before condemning the converter — they can trigger P0420.
- Inspect the exhaust for leaks around the manifold and oxygen-sensor bungs, especially ahead of the rear sensor.
- Compare upstream and downstream O2 sensor live data: a healthy rear sensor reads fairly steady. If it swings like the front sensor, the converter is likely worn out.
- Check the downstream O2 sensor and wiring for damage or contamination.
- Consider the converter's age and mileage — a high-mileage original cat is the usual culprit.
DIY vs shop cost
$200–$2,400
Parts + labour at an independent shop.
Make-specific notes
Very common on high-mileage Toyota and Lexus engines. OEM converters are expensive; a quality direct-fit aftermarket cat usually clears it once the rear O2 sensor checks out.
Often the original converter aging out around 120k–180k miles. Confirm there's no misfire before replacing the cat.
Check for exhaust leaks at the flex pipe and manifold first — a frequent P0420 trigger on Ford trucks and SUVs.
A known P0420 platform. Verify the rear O2 sensor and look for exhaust leaks before buying a converter.
P0420 questions, answered
Can I drive with a P0420 code?
Usually yes, for the short term. P0420 rarely affects how the car runs, but the problem won't fix itself and you'll likely fail an emissions test. Get it diagnosed within a few weeks.
Will a P0420 code clear itself?
It can disappear temporarily if it was triggered by a one-off event like a loose gas cap or a fouled sensor. If the converter is genuinely worn, the light returns after a few drive cycles.
Is P0420 always the catalytic converter?
No. About half the time it's the converter, but worn oxygen sensors, exhaust leaks, and untreated misfires or fuel-trim problems can all set P0420. Diagnose before replacing the cat.
How much does it cost to fix P0420?
If it's just a downstream oxygen sensor, expect $150–$300 at a shop. A catalytic converter replacement runs from around $400 for an aftermarket unit to $2,000+ for an OEM converter at a dealer.
Will a new catalytic converter fix P0420 for good?
Only if the converter was the real cause and any underlying issue is fixed. Replacing the cat while ignoring a misfire or rich fuel condition will ruin the new converter and bring the code back.